What Size Generator For 150 Amp Service?

Updated: January 28, 2026

A 150 Amp service panel can handle many large electrical devices. That’s why you will see the 150 Amp service panels in many older and newer homes. It has enough electricity to run a whole-house room heater, multiple air conditioners, a large refrigerator, and other small appliances at home.

However, you need to back up the 150 Amp service panel with the right generator during power outages. The key question is: do you need to back up everything, or just the essentials?

What Size Generator For 150 Amp Service Panel?

The short answer is 15kW to 22kW for most homes. But the actual size you need depends entirely on what you want to power during an outage.

Here’s the reality: a 150 amp panel can theoretically draw 36,000 watts (150A × 240V). But no home actually uses that much power simultaneously. Most households with 150 amp service use between 10,000 and 25,000 watts during peak demand.

Typical Loads in a 150 Amp Home

Add up what you need to run simultaneously. Most people prioritize refrigerator, lights, sump pump, and either heating or cooling. That’s typically 8,000-12,000 watts. Add the AC or electric heat, and you’re looking at 15,000-20,000 watts.

Do You Need to Back Up Everything?

Here’s what most generator dealers won’t tell you: backing up your entire 150 amp panel is expensive and often unnecessary.

You have two options:

Whole-house backup: A 22kW generator with a 150 amp automatic transfer switch powers your entire panel. Everything works as normal during an outage. This is convenient but costs more upfront and uses more fuel.

Essential circuits only: A 12-16kW generator with a load-shedding transfer switch powers only the circuits you choose. You select which breakers get backup power (refrigerator, lights, sump pump, one AC zone). This costs less and uses less fuel, but requires some planning.

For most 150 amp homes, a 16-18kW generator with load management handles everything you realistically need during an outage.

Generator Size Recommendations

How to Calculate Your Actual Load

The most accurate way to size your generator is to calculate your specific load. Here’s how:

Step 1: List every appliance you want to run during an outage.

Step 2: Find the wattage for each appliance. Check the nameplate on the device or use the table above as a guide.

Step 3: Add up all the running watts. This is your continuous load.

Step 4: Identify the largest starting wattage (usually the AC or well pump). Add this to your continuous load. This is your peak demand.

Step 5: Choose a generator that can handle your peak demand with 10-20% headroom.

Example: Refrigerator (150W) + Lights (100W) + Sump Pump (800W) + Furnace (500W) + Well Pump (1,000W) = 2,550W running. The well pump needs 2,000W to start, so peak demand is about 4,550W. A 6-8kW generator handles this easily. Add central AC (3,500W running, 7,000W starting), and you need 13-15kW.

150 Amp Service vs. Other Panel Sizes

If you’re comparing panel sizes, here’s how generator requirements typically scale:

A 100 amp service usually needs a 10-14kW generator. A 150 amp service needs 15-22kW. For 200 amp service, plan for 18-24kW or larger.

The panel size alone doesn’t determine generator size. Your actual electrical usage matters more. A 150 amp panel in a small home with gas appliances needs less backup power than a 100 amp panel in an all-electric home.

Common Mistakes When Sizing for 150 Amp Service

After helping hundreds of homeowners choose generators, I see the same mistakes repeatedly. Avoiding these can save you money and frustration.

Mistake 1: Matching generator size to panel size. A 150 amp panel doesn’t mean you need 36,000 watts of generator capacity. Your actual usage is what matters. Most 150 amp homes run fine on 16-20kW generators.

Mistake 2: Forgetting about starting watts. A 3-ton central AC unit runs on about 3,500 watts but needs 7,000+ watts to start. If your generator is sized only for running watts, it will trip when the AC kicks on. Always account for the largest motor startup in your load calculation.

Mistake 3: Oversizing to be safe. A 30kW generator for a 150 amp home wastes money on purchase price, installation, and fuel. Generators run most efficiently at 50-75% load. An oversized generator running at 20% load uses more fuel per watt and can develop carbon buildup.

Mistake 4: Ignoring fuel type. Natural gas generators produce about 10% less power than their rated capacity compared to propane. A 22kW natural gas generator delivers roughly 20kW. Factor this into your sizing if you’re using natural gas.

Mistake 5: Not considering future needs. If you’re planning to add an EV charger, hot tub, or workshop equipment, factor that into your generator size now. Adding capacity later is expensive.

Standby vs Portable Generator for 150 Amp Service

For a 150 amp service panel, you have two main options: a permanently installed standby generator or a large portable generator with a manual transfer switch.

Standby generators (14-24kW) connect permanently to your electrical panel and start automatically when power fails. They run on natural gas or propane, so you never need to store fuel. The automatic transfer switch handles everything – you might not even notice the power went out. Cost: ,000-,000 for the unit plus ,000-,000 for installation.

Large portable generators (10-15kW) cost less upfront (,500-,000) but require manual setup during each outage. You’ll need to wheel it outside, start it, and connect it to your transfer switch. They run on gasoline, which means storing fuel and refilling during extended outages. For a 150 amp panel, you’ll need at least a 10kW portable to power essential circuits.

The right choice depends on your situation. If you work from home, have medical equipment, or live in an area with frequent outages, a standby generator is worth the investment. If outages are rare and you’re comfortable with manual operation, a portable generator saves money.

Transfer Switch Options for 150 Amp Panels

The transfer switch is just as important as the generator itself. For a 150 amp service, you have several options:

150 amp whole-house automatic transfer switch: Powers your entire panel. When utility power fails, it automatically disconnects from the grid and connects to the generator. When power returns, it switches back. This requires a generator large enough to handle your full load (typically 20-24kW).

Load-shedding transfer switch: Automatically manages which circuits receive power based on available generator capacity. If you’re running the AC and the dryer starts, it can temporarily disconnect the dryer until the AC cycles off. This lets you use a smaller generator while still having access to large appliances.

Manual transfer switch with selected circuits: The most affordable option. An electrician connects your most important circuits (refrigerator, furnace, well pump, some lights) to a sub-panel. During an outage, you manually flip the switch to generator power. Works well with portable generators in the 7.5-12kW range.

Conclusion

For a 150 amp service panel, most homes need a generator between 15kW and 22kW. The exact size depends on whether you want whole-house backup or just essential circuits, and whether you have gas or electric appliances for heating, cooking, and hot water.

Start by listing what you actually need to run during an outage. Add up the wattages, account for starting surge on motors, and add 10-20% headroom. This gives you a more accurate size than simply matching your panel rating.

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• What size generator for 100 amp service?
• What size generator for 400 amp service?
• What will a 5500 watt generator run?

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